Bone in human and other mammal bodies is generally classified into two types, cortical bone, also known as compact bone, and trabecular bone, also known as cancellous or spongy bone. Cortical bone is much denser than trabecular bone with a porosity ranging between 5% and 10%. Cortical bone is found primarily in the shaft of long bones and forms the outer shell around trabecular bone at the end of joints and the vertebrae.
In the vertebrae, each bone is generally heart shaped, with spinous, inferior and superior processes joined to the vertebral body via opposing pedicles. To stabilize or fix deformities in the spine, implantable medical devices, known as spinal fixation devices, can be employed between adjacent vertebrae. These devices can be attached to the vertebrae using screws inserted through the pedicles (i.e., using “pedicle screws”) and other osseous structures such as the lamina and facet joints.
The outer shell of the pedicles is formed of dense cortical bone, which surrounds spongier trabecular bone. As mentioned above, trabecular bone is normally less dense than cortical bone. Degenerative conditions, which can result from diseases such as osteoporosis or injury, can cause the trabecular bone to weaken or degrade even further.
As a result of the lower density of the trabecular or compromised bone, screws of all sizes can loosen or shift position after implantation. Prior art attempts to secure bone screws in the pedicles include features to prevent loosening of the screws. These features can include deflectable wings that push outwardly against the bone, or toggle-bolt-like fittings that rotate once inside the cortical bone shell to prevent removal therefrom. Other prior art systems make use of cement or other binding agent to secure the bone screw inside of the pedicle.
These prior art solutions, however, are not without potential drawbacks. For example, prior art implementations are designed to provide permanent fixation of the bone screw within the pedicle or other bone. Accordingly, removal of these components requires a complicated and oftentimes invasive procedure. In addition, some prior art solutions utilize traditional threaded screws that require pre-implantation operations to correctly drill and tap a hole through the bone.
Finally, in certain situations it can be desirable to utilize radiolucent materials to avoid interference with medical imaging technologies. Prior art implementations, however, often utilize metal bone screws that are substantially radiopaque. This can be because radiolucent materials (e.g., any of various polymer-based materials) are often not suitable for creating fine features like threading on a screw.
Hence, there is a need in the art for a device and method for securing an anchor in degenerated bone such that the anchor exhibits enhanced bone purchase while remaining easy to remove after use. Further, there is a need in the art to design these anchors such that they can be formed from radiolucent materials to prevent interference with medical imaging technologies.